The L-A virus of the yeast Saccharomyces cerevisiae encodes its own major coat protein (Gag) and a multifunctional RNA polymerase - RNA biding protein (Pol) made as a Gag-Pol fusion protein formed by a -1 ribosomal frameshifting event identical in mechanism to that used by retroviruses of animal cells for a similar purpose. We have isolated host mutants that have increased levels of ribosomal frameshifting and several of these have lost ability to propagate the M1 satellite RNA. The mutants define eight mof (maintenance of frame) genes. Three mutants are ts for growth showing different cell cycle arrest phenotypes. We have described an antiviral system determined by the six chromosomal SKI genes. We now have evidence that this system works by inhibiting the translation of viral messages, apparently recognizing them as viral by their absence of a 5' cap structure or 3' polyA sequence. We have shown that a chimeric RNA - dependent RNA polymerase, made up of part of the L-A enzyme and part of the Sindbis virus polymerase, is capable of supporting the replication and transcription of the M1 satellite dsRNA in yeast. This supports the notion that our findings about the genetics of viral replication in yeast are applicable to mammalian viruses. We have evidence (in collaboration with the groups of Alasdair Steven, LSB, NIAMS, and Tim Baker, Purdue U.) that the structure of L-A virus particles is icosahedral with T-1 and an asymmetric unit consisting of a dimer of Gag protein molecules.